A drive for an extruder worm centered on an axis normally has a substantially closed housing axially juxtaposed with the worm, an input shaft extending along and rotatable in the housing about the axis and adapted to be coupled to a drive motor, an output shaft rotatable in the housing about the axis and adapted to be coupled directly to the worm, and an axial-thrust bearing engaged between the output shaft and the housing. A planetary transmission centered on the axis between the input and output shafts has a sun gear rotationally coupled to the input shaft, a ring gear spaced radially outward from the sun gear, a plurality of planet gears meshing with the sun and ring gears, and a planet carrier carrying the planet gears and rotationally coupled to the output shaft.
The extruder worm is provided internally with a heater or cooling unit whose feed lines extend axially out through the drive, so that the various parts form an axially throughgoing passage. Seals must be provided to segregate this passage from the housing interior, which is itself filled with oil to keep the various gears lubricated.
In such an arrangement (See "TPM Einschnecken-Extruder-Baureihe" of Thyssen Plastik Maschinen GmbH, Munich, 518 TPM 3D5.78 JWG 81324) the drive is of the single-stage type with the sun gear fixed on the drive shaft and the ring gear fixed in the housing. The planet carrier is supported solely by the planet gears, which are angularly equispaced about the axis between the sun and ring gears so that the carrier is effectively centered on the axis also. This planet carrier has an external toothing which engages in an internal toothed ring having an external toothing meshing with yet another internal toothing formed on a ring fixed on the output shaft.
The passage through the drive therefore axially traversing the input shaft, the output shaft, the axial thrust bearing, and the sun gear must be formed with respective sections of the passage. There must also, as mentioned above, be a seal between the output shaft and the sun gear, which seal is therefore located at the heart of the drive. Since the sun gear and output shaft rotate relative to each other, this seal is subjected to substantial wear and must be replaced periodically.
Such replacement is a fairly onerous job, as it entails substantial disassembly of the drive. To avoid taking apart the planetary transmission, the drive is normally unbolted from the extruder and its output end is removed along with the output shaft to gain access to this seal. This substantial disassembly can only be done once the heater and/or worm ejector is also taken off.